Flexible ball sockets have heretofore been used in ducting systems of aircraft. Typically, a bellows connection is used with the socket to provide the joint with flexible leakproof angulation, and the bellows is usually attached to the ball of the ball joint. Graphite or the like is often used to reduce friction between the ball and the socket and to reduce wear, as exemplified by the Rohr U.S. Pat. Nos. 2,840,394 and 2,502,753, the Alford U.S. Pat. No. 2,613,087, the Berry U.S. Pat. No. 1,769,905, the Gall, et al. U.S Pat. No. 1,349,060, the Murphy U.S. Pat. No. 1,925,335, the Walton U.S. Pat. No. 3,663,043, the Leak U.S. Pat. No. 1,155,495, and the Hallett U.S. Pat. No. 3,770,303.
In recent years, however, the operating temperatures of aircraft engines have increased along with their power and size. These changes in operating conditions have imposed new and more severe requirements on the ball joints. As a result, prior lubricants, such as graphite, Teflon sprays, and the like, have begun to break down, and this has increased friction and wear in the ball joint resulting in a shorter life. Another problem has arisen because the coefficient of friction of prior lubricants used in aircraft ducting has increased with temperature, making the ducting more rigid and less able to withstand vibration and acceleration and deceleration forces.
More recently, a material whose commercial name is Purbon.RTM., which is a treated form of graphite, has been found to have desirable low frictional properties over large extremes of operating temperatures in that its coefficient of friction tends to decrease with increasing temperature, e.g., up to the melting point of steel. This material appears to be suitable as a lubricant in flexible ball joints. However, Purbon is expensive and hard to work with, and it is desirable to use this material in an inexpensive easy-to-manufacture form.
It is also evident that aircraft design requires all parts, including flexible joints, to be as light as possible. To achieve this, the bellows is usually made very thin because in addition to being light, the thinness helps make the ball joint more flexible. However, this makes the bellows more susceptible to accidental blows or to high temperature failure which could result in a dangerous situation during flight.
What is needed, therefore, and comprises an important object of this invention, is to provide an inexpensive lightweight ball joint utilizing Purbon as the lubricant between the ball and the socket to reduce friction and insure that the ducting remains flexible over a wide range of operating temperatures.